Mesoporous Octagonal Microplates Co2–xNixP2O7(x = 0.00, 0.50, 1.00, 1.50, and 2.00) Metal Pyrophosphates for High-Performance Asymmetric Supercapacitors

Abstract

Monoclinic Co2–xNixP2O7(x = 0.00–2.00) pyrophosphates were synthesized and composition-tuned to reveal an optimal morphology/porosity at x = 1.00 that delivered high-performance supercapacitor electrodes. Across the series, X-ray diffraction (XRD) results confirmed a pure phase of Co2–xNixP2O7(P21/c), with Ni substitution providing acceptable crystallite-size shifts and a systematic lattice shrinkage. Field emission scanning electron microscopy (FE-SEM) showed that x = 1.00 specimen formed well-faceted octagonal microplates with the highest specific surface area (11.381 m2/g) and mesoporous surfaces with average pore sizes of ∼10 nm and mesopore volume of 0.0909 cm3/g, as revealed by Brunauer–Emmett–Teller/Barett–Joyner–Halenda (BET/BJH) analysis. X-ray photoelectron spectroscopy (XPS) identified Co2+, Ni2+, and P5+, which is consistent with OH–-coupled M2+/M3+pseudocapacitance observed using cyclic voltammetry (CV)/galvanostatic charge–discharge (GCD) in 3 M KOH. The x = 1.00 electrode achieved 654 F/g at 0.5 A/g and maintained 84.6% of its initial state after a 3000-cycle GCD test at 5 A/g. An asymmetric device (Co1.00Ni1.00P2O7//rGO) delivered 56.68 Wh/kg at 938.36 W/kg. Electrochemical enhancement resulted from a combination of mixed-metal redox centers and optimized meso-porosity/microstructure, as evidenced by CV, GCD, and electrochemical impedance spectroscopy (EIS). The findings demonstrated that compositional control presented an effective strategy for controlling mesoporousity and enhancing redox utilization in Co–Ni pyrophosphate electrodes.